Conventionally, such decontamination method by laser is known as irradiating substantially parallel light pulse laser beams of lower average power output onto the surface of the objects contaminated by the RIs with such objects linearly or planarly scanned, the device employing which method is characterized in that the contaminants deriving from RIs, which are black and others in color and higher in photoabsorption, are vaporized for removal without doing damage on the base metal and as such.
However, according to the above method by which the pulse laser beams are rendered into parallel light, making the condensed area smaller for the purpose of enhancing the power density (irradiation intensity) unavoidably leads to causing uneven irradiation, so that the same spot shall be irradiated several times. On the other hand, enlarging an irradiation area of one pulse causes the power density to be lowered (in the order of some MW/cm2 or less) so that the irradiated beams result in being reflected from the lustrous metallic surface, with the result that the RIs advanced deeply into the microscopic cracks on the metallic surface cannot be removed at all.
Further, conventionally, besides the above-mentioned laser decontamination, such decontamination employing a sandblast, a sander and a grinder is carried out, according to which mechanical methods in order to remove the attached RIs, it is general that a portion of the surface of the object contaminated by them is scraped off by the thickness ranging from 0.05 mm to 0.1 mm or more
However, according to the above-mentioned mechanical methods, during the grinding not only the RIs are ready to reenter the surface so as to cause recontamination, but also the grinding device (such as the nozzle of the sandblast) is secondarily contaminated by the grinding particles of the sandblast, the grinding belt of the sander or grinder or the grinding disk that are used repeatedly.
Further, providing that the sandblasts and sanders secondarily contaminated by the grinding particles or belt are exchanged with new ones every time the decontaminations are carried out, it makes the cost incurred for exchanging such mechanical devices and their parts so bulky that not only the decontamination cost goes overboard, but also a large volume of secondary wastes are produced, which is unfavorable in view of the cost-saving aspect and the eco-friendly trend.
On the other hand, conventionally, such method is known as the objects contaminated by the RIs being clipped into the solution containing an oxidant and a reductant so as to make the contaminants solved into the solution, according to which chemical method it requires a lot of disposal cost to dispose with a large volume of ion-exchange resins used for separating the RIs from the wasted solution, which resins are burnt so as to be reduced in volume and stabilized with concrete and the like for storage.
Further, in recent years, such technique is proposed as the pulsed laser beams whose peak output is 10 MW or higher (for instance, refer to Document 1) being employed, according to which technique it is unable to focus the laser beams on the surface of the object contaminated by the RIs with precision, so that the power density (irradiation intensity) in the order of GW/cm2 cannot be secured. Moreover, uneven irradiation density might happen on the surface of the object with irregularities.
Furthermore, where such high-output pulsed laser beams are employed, it requires that the thermally induced diffusion and reattachment of the RIs be taken into due account, besides, with the method by which the laser beams are irradiated with the laser head moved, because there is limit in the scanning velocity for the low-velocity motion even when it might be automatically scanned, the thermally induced diffusion of the RIs is aggravated in which the portions onto which the beams are irradiated result in being extensively and deeply fused and thermally affected. Such method is also known (refer to non-patent literature 1) as fusing the surface of the object by CW (Continuous Wave) laser beams of high output power and removing the fused portions by a pressurized gas, which method is also subjected to aggravated thermal effect so as to make the thermally induced diffusion of the RIs further aggravated.
On the other hand, a laser machining device in which a focal length is adjusted with a beam expander in use is also well known (refer to Patent Literature 2), but with such device, the condensing optical system including compound lenses are disposed posterior to the XY scanner, so that it often happens that measuring a surface shape with a laser range finder fails because the reflected light from the object is under the influences of such compound lenses and as such. Further, passing the machining and measuring laser beam through the compound lenses and as such makes such beam greatly attenuated, so that the available machining laser beam unavoidably results in being smaller in output power.